1. Field of the Invention
The present invention relates to a superconductive device manufacturing method, and more specifically to a method of manufacturing a superconductive device by use of a plasma etching process.
2. Description of the Prior Art
As the etching method adopted to process a YBCO superconductive thin film of a superconductive device, various etching methods are so far well known, such as wet etching, laser etching, dry etching, focused ion beam etching, etc.
Here, a method of processing a YBCO superconductive thin film by use of dry etching is disclosed in Hiroshi Sato, Hiroshi Akoh, Keirou Nishimura, Masahiro Aoyagi and Susumu Takada; Journal of Japan Applied Physics Society, 31, (1992) L1044, for instance. In this processing method, as shown in FIG. 11, after Ar gas has been introduced into an ECR (electron cyclotron resonance) chamber 1101, an ECR plasma is generated by use of a microwave, and an Ar ion beam is extracted from the generated ECR plasma into an etching chamber 1102. Further, by use of the extracted Ar ion beam, the YBCO superconductive thin film formed on a sample 1104 held by a sample holder 1103 is etched. In this Paper, the acceleration voltage of the Ar ions is 240 V, and the ion current density is 0.15 mA/cm.sup.2. Further, as an etching mask, an ordinary photoresist is used.
Here, when the YBCO superconductive thin film is being etched, the temperature on the processed surface rises locally. Therefore, oxygen is inevitably desorbed from the processed surface, so that the superconductivity characteristics deteriorate. To overcome this problem, in this instrument as shown in FIG. 11, the sample 1104 is cooled by use of liquid nitrogen; that is, the sample temperature is reduced to 77 K during etching to suppress the desorption of oxygen.
FIG. 12 shows an evaluation result of the relationship between the sample temperature during etching and the superconductivity characteristics. In more detail, FIG. 12 shows the relationship between a line width W (.mu.m) (on the abscissa) obtained by patterning the YBCO superconductive thin film into a line shape and the critical current density Jc (A/cm.sup.2) (on the ordinate), in which circular marks denote the values obtained when the sample 1104 is cooled down to 77 K by liquid nitrogen, and triangular marks denote the values obtained when the sample is cooled to 5.degree. C. by water. FIG. 12 indicates that the superconductivity characteristics are excellent when the sample 1104 is cooled down to 77 K by liquid nitrogen, as compared with when cooled to 5.degree. C. by water.
Further, another method of processing the YBCO superconductive thin film by use of dry etching is disclosed in Roland Barth, Bernd Spangenberg, Christian Jaekel, Hartmut G, Roskos, and Heinrich Kurz; Applied. Physics. Lett. 63, (1993) 1149, for instance. In this processing method, first a PMMA film is patterned by use of EBL (electron beam lithography); secondly a Ti film is patterned by RIE (reactive ion etching) with this PMMA film as a pattern mask; and the YBCO superconductive thin film is etched by sputtering Ar and oxygen ions with this Ti film as a mask (the acceleration voltage is 420 V and the sample temperature is 77 K). Further, after etching, the thin film is plasma-oxidized by use of an oxygen plasma, to restore oxygen desorbed from the processed surface during etching.
FIG. 13 is a graphical representation showing the comparison results of the superconductivity characteristics between before plasma oxidization and after plasma oxidization with respect to the line widths W of the YBCO superconductive thin film bridge (length: 10 .mu.m). FIG. 13 indicates that when the plasma oxidization is made, the superconductivity characteristics of the bridge with a line width of 0.4 .mu.m can be increased by about one order of magnitude, as compared with omission of plasma oxidization.
On the other hand, in the case where the superconductive device is a superconductive high frequency device, as the method of processing the YBCO superconductive thin film, an ion beam etching method using photolithography and a laser etching method are both so far well known.
The processing methods by use of these etchings are disclosed in Tsuyoshi Takenaka, Shuichi Fujino, Keiich Yamaguchi, Kunihiro Hayashi, and Katsumi Suzuki; Proceedings of 5th International Symposium on Superconductivity, Nov. 16-19, 1992, Kobe, Japan, for instance.
In this paper, after the YBCO superconductive thin film is formed on a MgO substrate with a thickness of 0.5 mm, this YBCO superconductive thin film is processed by etching, to manufacture a microstrip line resonator of 13.5 GHz. Further, this paper compares the characteristics of the resonator between when an ion beam etching using a photoresist is adopted and when a direct KrF excimer laser etching (without use of any photoresist) is adopted.
FIG. 14 is a graphical representation showing the relationship between the unloaded Q value of the resonator and the temperature obtained on the basis of the experiments, in which circular marks denote the values obtained when the ion beam etching method is adopted and cross marks denote the values obtained when the laser etching is adopted. FIG. 14 indicates that the dependency of the unloaded Q value upon the temperature is almost the same between the two cases. Therefore, this indicates that the laser etching method is advantageous to manufacture the superconductive high frequency device, because the photoresist process is not required.
As described above, when the YBCO superconductive thin film is etched, it is necessary to eliminate such a disadvantage that oxygen is desorbed from the etched surface due to a temperature rise, because the superconductivity characteristics deteriorate. Therefore, conventionally, in order to prevent oxygen from being desorbed from the etched surface, that is, to prevent the deterioration of the superconductivity characteristics, the sample is cooled by use of liquid nitrogen, or else oxygen is supplied to the etched surface by plasma oxidization after etching.
In the method of cooling the sample by use of liquid nitrogen, however, since the sample is cooled to 77 K by the liquid nitrogen and then heated to room temperature after etching, there exists a problem in that a long manufacturing time is inevitably needed.
Further, in the method of supplying oxygen onto the etched surface by plasma oxidization after etching, since an additional process is also required, there exists the similar problem in that the number of the manufacturing processes increases.
In addition, in these prior art methods, as understood with reference to FIGS. 12 and 13, although it is possible to prevent the deterioration of the superconductivity characteristics with respect to a wide pattern line formed by the etching, when the line having a width as narrow as several .mu.m or less is etched, the superconductivity deteriorates inevitably at an edge of the narrow line, so that a sufficient effect cannot be obtained.
In particular, in the case of the superconductive high frequency device, since the current is generally concentrated at the edge of the line, the quality of the line edge exerts a serious influence upon the device characteristics. Therefore, there exists a strong need of preventing oxygen desorption perfectly during etching process. For instance, in the above-mentioned Paper related to laser etching (Proceedings of 5th International Symposium on Superconductivity, Nov. 16-19, 1992, Kobe, Japan), it is disclosed that the melted YBCO with a width of 1 .mu.m and a height of 1 .mu.m is laminated on the line edge. In this case, it is apparent that the melted YBCO exerts a harmful influence upon the device characteristics.